We would suggest a medium level of confidence regarding "what is happening now". This applies to non-native species overall as the information available in the UK is moderate, and consensus is also only moderate.

One of the major problems of assessing the potential impact of climate change on non-native species is the lack of knowledge regarding where many of the species are established. There has been no full scale base-line survey of the presence of non-native species in the marine environment so the current distribution of many species is not known.

There is an urgent requirement for monitoring of the range of and effects of climate change on, established invaders. Only then can detailed risk assessments and contingency plans be prepared for future invaders. Further, the question of how climate change will interact with other ecological pressures (such as invasive species or habitat fragmentation) to create synergistic effects also needs to be considered (Sutherland et al. , 2006).

Commercial impacts

Commercially, some economically important species have been introduced, but some associated pests and parasites adversely affecting native species have also been unintentionally introduced. Control methods, where applied to nuisance species, are fairly ineffective and no non-native marine species have yet been successfully eradicated from British waters. Of the species deliberately introduced for aquaculture, only a few bivalve molluscs have become established in the natural environment beyond the confines of their cultivation. For example Crepidula fornicata has become a dominant mollusc in estuaries on the south coast and especially in the Solent, outcompeting oysters.

The Pacific oyster, Crassostrea gigas , is an important invasive species in its own right because it is extensively cultivated in Scotland. Cultivation of this species is controlled in that it occurs in containment (i.e. on trays or in bags) and is only allowed to go ahead after a consultation process. It is assumed that the low temperature of Scottish waters would mean that this species would be unable to establish itself. There has been no successful spat fall recorded in Scottish waters although maturation of the gonad and the occasional release has been noted but not settlement or establishment of populations. As this species has become established in other countries such as the Netherlands and Germany as well as areas in the south of the UK there is potential for an increased risk of the species becoming established in Scotland as water temperatures increase. This could lead to the out-competition of native filter feeders (Eno et al. , 1997) and the extensive modification of estuarine habitats.